have you thought of using Google adwords? you can limit the budget to your means, as low as $20 a month, and specify which search words will prompt the ad to appear for the person searching. not that it worked for my own site, but i have only a general chat forum, games etc. so was hard to actually think of key words.. it didn't work too well for me, but it just might for you here.. my own budget was limited to $20, and i never did pay much more than $16 a month from memory. might be worth a few months trial perhaps?

have you been looking at vbulletin.org for ready made styles and coloured buttons? there's various mods and things too.. i had fun colouring buttons as i couldn't find anything in purple